1. Field of the Invention
The present invention concerns devices for generating braking pressure for a braking installation incorporating at least two braking circuits, particularly for a motor vehicle, incorporating a casing enclosing a working chamber intended to be joined to a first braking circuit, a sliding actuating piston having a first end exposed to the working chamber and defining, by its other end, a braking pressure chamber intended to be joined to a braking circuit, a control valve means for admitting selectively a fluid under pressure coming from a source of fluid under pressure into the working chamber, and an actuating plunger intended to be joined to a brake pedal, protecting into the working chamber and able to be coupled mechanically with the first end of the actuating piston and with the valve means.
2. Description of the Prior Art
A device of this type is described, for example, in the documents U.S. Pat. No. 2,410,269 and FR-A-2,332,445. In this type of brake servomotor, the pressure modulated by the valve means and admitted into the working chamber is transmitted directly to the first braking circuit in a hydrodynamic mode, more commonly known by the Anglo-Saxon term "full power" mode, this same modulated pressure acting on the actuating piston so as to create, this time in a hydrostatic mode, in the braking pressure chamber, an amplified pressure which is transmitted to the second braking circuit. These devices therefore operate in a mixed mode, differently for each braking circuit. However, in the case of failure of the pressure source, or of the the associated supply circuit, only the second braking circuit remains operative, by operating "manually" under the effect of the muscular force exerted by the driver. Devices with purely hydrodynamic operation also exist for the two braking circuits, as described for example in the document FR-A-2,457,202. In the latter case, failure of the pressure source renders the braking installation completely inoperative.